Hardening

A plantlet that has developed entirely inside a culture vessel has grown under artificially high humidity, a controlled temperature and limited light, and has often relied partly on sugars in its medium rather than its own photosynthesis - conditions nothing like the outside world it must eventually survive in. Hardening is the gradual process of acclimatising such an in vitro plantlet to normal environmental conditions: it is first transferred to a greenhouse or a dedicated hardening chamber, where it is exposed step by step to humid conditions under diffused (rather than direct) light, before finally being moved out into normal field conditions. Skipping or rushing hardening is one of the most common reasons an otherwise successful tissue culture plantlet fails to survive transplantation, so it is treated as …